A conventional overhead-valve internal combustion engine for a motorcycle or an aircraft includes a crankcase and at least one cylinder, cylinder head and rocker arm box. The cylinder is typically secured to the crankcase by studs or bolts extending through a flange at the base of the cylinder and threaded into the crankcase, while the cylinder head may be separately bolted to the cylinder, or the studs or bolts may be threaded into the crankcase and extend through both cylinder and head.
As is well known, the engine's camshafts are conventionally located in the crankcase, and some form of cam follower imparts motion to pushrods and thence to rocker arms in a rocker arm box located above the head. The pushrods are generally situated outwardly of the cylinder and typically farther beyond the cylinder than the conventional securing studs or bolts, and may extend from a tappet or lifter block secured to the crankcase. When the pushrods are moved by operation of the cam in the crankcase, a force is applied to the rocker arm sufficient to overcome the inertia of the valve, rocker arm, valve collar and keeper as well as to compress the valve spring. The effect is that the force generated tries to lift the rocker arm and box away from the head. The cylinder and head bolts, particularly on the pushrod side of the cylinder head, must resist these forces as well as the combustion pressure within the cylinder.